The invention relates to a process for liquefying and subcooling a hydrocarbon-rich fraction, particularly of natural gas, wherein, once cooled down, the fraction is subjected to a partial condensation to remove heavy hydrocarbons, particularly benzene.
Liquefaction and subcooling of a hydrocarbon-rich fraction is typically achieved against at least one refrigerant cycle and/or at least one mixed refrigerant cycle.
Preventing outages caused by freezing-out of certain components of the fraction to be liquefied is of great importance in the liquefaction of hydrocarbon-rich fractions, particularly natural gas. Water and carbon dioxide are typically removed at the beginning of the process at ambient temperature by chemical scrubbing (e.g. amine scrubbing) and/or adsorptive processes to such an extent that they do not cause undesired solid formation during liquefaction of the hydrocarbon-rich fraction.
Freezing-prone heavy hydrocarbons (HH) (hereinbelow the term “heavy hydrocarbons” is to encompass C6+ hydrocarbons), benzene in particular, can be removed under ambient conditions from the fraction to be liquefied only at great cost and inconvenience. Hence it is common practice to subject the feed gas to a slight partial condensation and then draw off an HH-rich liquid fraction in a separator to sufficiently reduce the risk that the gas phase exiting this separator will freeze during subsequent liquefaction and subcooling.
However, partial condensation generally only ensures that the gas phase is sufficiently depleted in HHs, particularly benzene, when the gas mixture to be liquefied comprises components having a middle boiling range, for example propane, butane and/or pentane, which during cooling-down of the feed gas undergo liquefaction in sufficient amounts before the HHs and thus act as solvent for said HHs.
When an insufficient concentration of middle boilers—this is referred to as so-called lean gas—in the composition of the feed gas does not allow sufficient depletion in benzene (typically to <1 ppmv) by partial condensation and subsequent removal of the HH-rich liquid, unwanted freezing-out can still occur.
It is an object of the present invention to specify a process of the type in question for liquefying and subcooling a hydrocarbon-rich fraction, particularly of natural gas, which achieves reliable and economical removal of heavy hydrocarbons even under these conditions.